1. Field of the Invention
This invention relates to a novel and improved anti-lock control system for motor vehicles, which is operative to prevent the wheels of the motor vehicle from being locked during braking operation of the motor vehicle.
2. Description of the Prior Art
Generally, with an anti-lock control system for motor vehicles, anti-lock control is effected by means of microcomputers such that hold valves and decay vaLves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheel speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake hydraulic pressure, for the purpose of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance.
FIG. 1 of the accompanying drawings illustrate, by way of example, manners in which wheel speed Vw, wheel acceleration and deceleration +Vw and -Vw, and brake hydraulic pressure Pw are varied during the operation of the conventional anti-lock control system, together with hold signal HS and decay signal DS for opening and closing hold valves and decay valves, as disclosed in U.S. Pat. No. 4,741,580.
When the brake equipment of the motor vehicle is not being operated while the motor vehicle is running, the hold valves remain open while the decay valves remain closed, the brake hydraulic pressure Pw is not increased; and when the brake equipment is operated, the brake hydraulic pressure Pw is rapidly increased at time t0 so that the wheel speed Vw is decreasd (normal mode). A reference wheel speed Vt is set up which is lower by a predetermined amount .DELTA.V than the wheel speed Vw and follows the latter with such a speed difference. More specifically, reference wheel speed VT is set up so that when the deceleration (negative acceleration) -Vw of the wheel reaches a predetermined threshold level, say -1.0 G at time t1, anti-lock control is started, and the reference wheel speed VT is thereafter made to linearly decrease with a deceleration gradient .theta. (=-1.0 G). At time t2 when the deceleration -Vw ofthe wheel reaches a predetermined maximum value -Gmax, the hold signal HS is generated so that the hold valves are closed, thus holding the brake hydraulic pressure Pw.
With the brake hydraulic pressure Pw being held, the wheel speed Vw is further decreased. At time t3, the wheel speed Vw and the reference wheel speed VT become equal to each other, and a first cycle of anti-lock control is started; and the decay signal DS is generated, by which the decay valves are opened so that reduction of the brake hydraulic pressure Pw is started. As a result of this reduction of the brake hydraulic pressure Pw, the wheel speed Vw is changed from increasing to dcreasing at time 14 when a low peak VL of the wheel speed Vw occurs. Either at the time t4 or at time t5 when the wheel speed Vw is increased up to the level of a speed Vb that is higher than the low peak speed VL by 10% of the difference Y between wheel speed Va occurring at the time t3 when the reduction of the brake hydraulic pressure was started and the low peak speed VL, i.e., Vb=VL+0.1Y, the decay signal DS is interrupted (FIG. 1 shows the case where the decay signal DS is interrupted at the time t5), and as a result the decay valves are closed so that the reduction of the brake hydraulic pressure Pw is stopped and thus the brake hydraulic pressure is held. The wheel speed Vw is further increased up the level of a speed Vc that is higher than the low peak speed VL by 90% of the difference Y between the wheel speed Va occurring at the time t3 when the reduction of the brake hydraulic pressure Pw was started and the low peak speed VL, i. e., Vc=VL+0.9Y. Subsequently, at time t7, a high peak VH of the wheel speed VW is reached; thereupon, the brake hydraulic pressure Pw is again increased. In this case, the buildup of the brake hydraulic pressure Pw is effected in such a manner that the brake hydraulic pressure Pw is alternately increased and held in succession by the fact that the hold signal HS is turned on and off mincingly, or with relatively short intervals so that the brake hydraulic pressure Pw is caused to gradually build up. In this way, the wheel speed Vw is decreasd, and at time t8 (corresponding to the time t3), a second cycle of the mode for reduction of the brake hydraulic pressure occurs. The time period Tx of the initial brake hydraulic pressure buildup occurring at the time t7 is determined on the basis of calculation of the average acceleration (Vc-Vb)/.DELTA.T over the time interval .DELTA.T between the time t5 and the time t6 (the average acceleration depends on the friction coefficient .mu. of the road surface), and the time period ofthe subsequent pressure holding or pressure buildup is determined on the basis of the acceleration or deceleration of the wheel which is detected immediately prior to the pressure holding or pressure buildup. The brake hydraulic pressure increasing, holding and reducing modes are effected in combination as mentioned above, and thus the wheel speed Vw can be controlled so that the vehicle speed can be decreased, while the wheels of the motor vehicle are prevented from being locked.
Referring to FIG. 2, there is shown a conventional X-type two-channel brake apparatus wherein the righthand front wheel and lefthand rear wheel are controlled through a first oil hydraulic piping channel Q1 common thereto while the lefthand front wheel and righthand rear wheel are controlled through a second oil hydraulic piping channel Q2 common thereto (two-channel control system). In an attempt to apply the above-described anti-lock control system to a motor vehicle which is commonly of the front-engine, front-drive (FF) type and incorporates such an X-type two-channel brake apparatus as shown in FIG. 2, it has heretofore been the practice that anti-lock control with respect to the respective oil hydraulic piping channels Q1 and Q2 is effected through independent control channels respectively(two-channel control). In such a case, with a two-sensor, two-channel type system wherein two speed sensors are provided in association with the lefthand and righthand front wheels respectively, the lefthand and righthand front wheel speeds are regarded as the speeds of the wheels belonging to the respective channels (referred to as channel speeds Vs1 and Vs2 hereinafter), and anti-lock control with respect to the respective channels is effected on the basis of the channel speeds Vs1 and Vs2. With a four-sensor, two-channel type system wherein four speed sensors are provided in association with all the four wheels respectively, anti-lock control with respect to the respective channels is effected on the basis of a first channel speed Vs1 which is the lower one of the righthand front wheel speed and higher one of the righthand and lefthand rear wheel speeds and a second channel speed Vs2 which is the lower one of the lefthand front wheel speed and the selected higher rear wheel speed.
In the arrangement shown in FIG. 2, pressure control valves PCV1 and PCV2 are provided at the rear-wheel side sections of the oil hydraulic channels Q1 and Q2 respectively in such a manner that in the course of brake oil pressure buildup, the proportion of the braking force imparted to the rear wheels is made to be lower than the proportion of the braking force applied to the front wheels. In FIG. 2, where are also provided a master cylinder MC, and a modulator MOD including the hold valves and decay valves associated with the respective channels.
With the two-channel anti-lock control system as mentioned above, if quick braking is applied when the motor vehicle is running on a road surface which represents remarkably different friction coefficients (so-called "split friction (.mu.)" road surface), then a significant torque unbalance tends to occur between the lefthand and righthand wheels at the first cycle of the anti-lock control so that a great yawing moment tends to occur at the beginning of the anti-lock control, thus resulting in an unstable behavior of the motor vehicle.